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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 6 |
| 3 | 33 |
| 4 | 168 |
| 5 | 984 |
| 6 | 5640 |
| 7 | 37440 |
| 8 | 246240 |
| 9 | 1853280 |
| 10 | 13880160 |
| 11 | 117391680 |
| 12 | 989936640 |
| 13 | 9315855360 |
| 14 | 87500528640 |
| 15 | 907925760000 |
| 16 | 9408462336000 |
| 17 | 106785133056000 |
| 18 | 1210848984576000 |
| 19 | 14928525545472000 |
| 20 | 183922359312384000 |
| 21 | 2448351304261632000 |
[1,6,33,168,984,5640,37440,246240,1853280, 13880160,117391680,989936640,9315855360, 87500528640,907925760000,9408462336000, 106785133056000,1210848984576000, 14928525545472000,183922359312384000, 2448351304261632000] |